A logic bomb is a
piece of software that sits dormant for a period of time until
some event causes its malicious payload to be implemented.

One definition for a time bomb is

A time bomb is a
piece of software that sits dormant for a period of time until
some specific date and/or time causes its malicious payload to
be implemented.

Some people use the term logic bomb to refer to
both logic and time bombs.

Logic Bombs.
I want to take a little bit of time to write some more about logic
bombs.

Some examples of situations where logic bombs
might occur are

an intruder leaves behind some things
to ensure that any traces they have left of their intrusion
are wiped out if discovered

a disgruntled employee causes certain
records to be deleted when and if they are terminated

Some examples of logic bombs are

Time Bombs.
I want to take a little bit of time to write some more about time
bombs.

Some examples of time bombs are

Michelangelo Virus - which was set to
go off on March 6 - the birthday of the Renaissance
sculptor/painter.

it was designed to delete
particular data from user's hard drives

Chernobyl Virus - which was set to go
off on April 26 - the date of the Chernobyl nuclear disaster

attempted to rewrite the victim's
BIOS

attempted to erase the victim's
hard drive

Blaster - attempted an attack on
windowupdate.com

Code Red - attempted attack on the
United State's White House website

Protection.
The single best approach to protecting your system from bombs is to
never download an/or install that has uncertain sources, security
and integrity. A good up-to-date virus scanning program should
help considerably with detecting known bombs that have gotten
installed on a system. Such virus scanning software should
also help to detect and prevent installation of bombs.